"What we [in the Southern Hemisphere] call a species is different from what people in the Northern Hemisphere are calling a species in terms of how long they've been around," says Shine.

He says more extreme conditions in the Northern Hemisphere would have wiped out species there, whereas species in the less extreme south would have been able to better survive.

If this was the case, species from warmer refuges would later reinvade these vacant northern area, says Shine, and tend to be younger than similar species in the south.

Hypothesis investigated

Shine and Dubey investigated whether there was evidence to support this hypothesis.

"We should see that, on average, a species living in Australia has been there for longer than a species living, say, in Sweden," says Shine.

Shine and Dubey took advantage of the fact that the longer a species is in a particular location, the more genetically diverse its populations.

Conversely, populations of a species that has arrived in an area relatively recently, for example from a refuge, would have a lower amount of genetic diversity.

The researchers studied how the DNA sequences of a range of reptile and amphibian species changed over time, and calculated the date of the oldest ancestor of each species.

"There turns out to be a consistent difference in that number," says Shine.

He and Dubey found that, on average, the common ancestors of species in the Southern Hemisphere are much older than those of species in the Northern Hemisphere.

The average for frog species in the north is about 2 million years old whereas in the south the average [age] is 4.5 million years, says Shine.

"For turtles it's about 2 million years versus about 10 million years. So there are quite big differences."

Implications for conservation

Shine says the findings should be taken into consideration when planning for conservation.

He says the relatively recent "colonising" animals of the north are perhaps more flexible and better able to deal with new challenges, such as climate change.

By comparison, systems in southern continents like Australia have become more "intimately co-adapted", says Shine.

"Species that come from very old lineages that have been around for a long time seem to become rather set in their ways and don't seem to be quite as good at dealing with rapid change," he says.

"If you evolved for many millions of years in the same place you gradually [develop] a set of adaptations to living just exactly there, and you're not quite so good at dealing with things as they change."